Secondary battery and electric device

Abstract

A secondary battery and an electric device. The secondary battery comprises a negative electrode sheet; the negative electrode sheet comprises a negative electrode current collector and a negative electrode film layer provided on at least one surface of the negative electrode current collector; the negative electrode film layer comprises a negative electrode active material and a conductive agent; the negative electrode active material comprises a graphite material; the tap density of the graphite material is greater than or equal to 1.2 g / cm3; and on the basis of the total mass of the negative electrode film layer, the mass fraction of the conductive agent is greater than or equal to 1.6%. The cycle stability of the secondary battery is further improved.

Description

Secondary batteries and electrical devices

[0001] Cross-references

[0002] This disclosure refers to Chinese patent application No. 202311679355.3, filed on December 7, 2023, entitled “Secondary Battery and Electrical Device,” which is incorporated herein by reference in its entirety. Technical Field

[0003] The present disclosure belongs to the field of battery technology, and particularly relates to a secondary battery and an electrical device. Background Art

[0004] In recent years, secondary batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power stations, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace and other fields.

[0005] As the requirements for the endurance of electrical devices continue to increase, more stringent requirements are placed on the cycle life of secondary batteries. How to further improve the cycle life of secondary batteries is a technical problem that those skilled in the art urgently need to solve.

[0006] Summary of the Invention

[0007] An object of the present disclosure is to provide a secondary battery and an electric device, which can further improve the cycle stability of the secondary battery.

[0008] The present disclosure provides a secondary battery comprising a negative electrode plate, the negative electrode plate comprising a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector, the negative electrode film layer comprising a negative electrode active material and a conductive agent, the negative electrode active material comprising a graphite material, and the tap density of the graphite material being greater than or equal to 1.2 g / cm 3 ; and based on the total mass of the negative electrode film layer, the mass proportion of the conductive agent is greater than or equal to 1.6%.

[0009] By including a relatively high content of conductive agent in the negative electrode film layer containing high tap density, the electrical contact between graphite materials can be effectively enhanced, the degree of black spots generated at the negative electrode can be reduced, the advantage of high tap density graphite material in terms of high cycle life can be fully utilized, and the cycle stability of the battery can be improved.

[0010] In any embodiment of the present disclosure, based on the total mass of the negative electrode film layer, the mass proportion of the conductive agent is 1.6%-2.5%, and can be optionally 1.65%-2.0%.

[0011] Conductive agents within a suitable range can enhance the electrical contact between graphite materials while enabling the slurry to be effectively dispersed. Excessive loss of lithium ions during storage due to excessive amounts of highly active conductive agents in the slurry will not occur, thereby reducing the probability of side reactions occurring during the cycle and further improving the cycle stability, storage performance, and especially high-temperature storage performance of the secondary battery.

[0012] In any embodiment of the present disclosure, the tap density of the graphite material is 1.20 g / cm 3 -1.42g / cm 3 , optional 1.25g / cm 3 -1.35g / cm 3 .

[0013] A tap density within an appropriate range can improve the cycle stability of the secondary battery while reducing the probability of black spots on the negative electrode sheet and improving the quality of the negative electrode sheet.

[0014] In any embodiment of the present disclosure, the graphite material includes primary particles; optionally, based on the total number of particles of the graphite material, the number of primary particles in the graphite material accounts for greater than or equal to 85%.

[0015] Compared with secondary particles, primary particles can effectively reduce the grain boundary content, reduce the probability of side reactions between graphite materials and electrolytes, improve the uniformity of the electrodes, and further improve the cycle stability of secondary batteries.

[0016] In any embodiment of the present disclosure, Dv1 of the graphite material is 1.0 μm-4.0 μm, and optionally 1.5 μm-3.0 μm.

[0017] Applicants have discovered that the Dv1 particle size of graphite materials has a significant impact on the electrochemical performance of secondary batteries. A graphite material with a Dv1 within an appropriate range not only allows the negative electrode film layer to contain a certain proportion of fine powder, namely, small-particle graphite material, which is beneficial for improving the electrical contact of the negative electrode film layer, improving dynamic performance, reducing the severity of negative electrode black spots during battery cycling, and reducing the probability of battery electrochemical performance "diving." It also reduces the probability of small-particle graphite material clogging the pores of the negative electrode film layer, maintaining normal electrode pores, allowing for high electrolyte wettability, reducing the risk of lithium plating at the negative electrode, and further improving the battery's cycling stability.

[0018] In any embodiment of the present disclosure, the volume distribution particle size Dv50 of the graphite material is 10 μm-18 μm, and optionally 12 μm-16 μm.

[0019] Graphite materials with Dv50 within the above range can easily form effective connections in the negative electrode film layer to improve the conductivity of active ions and electrons; they can also form a reasonable pore structure in the negative electrode film layer, thereby comprehensively improving the cycle stability of the secondary battery.

[0020] In any embodiment of the present disclosure, the particle size distribution of the graphite material (Dv90-Dv10) / Dv50 is 1.20-1.70, and optionally 1.35-1.60.

[0021] The particle size distribution of the graphite material is within the above range, which is beneficial to reducing the bridging phenomenon between the graphite materials and improving the electrical contact of the negative electrode film layer, and is also beneficial to improving the uniformity of the dispersion of the graphite material in the slurry, thereby comprehensively improving the cycle performance of the battery.

[0022] In any embodiment of the present disclosure, the specific surface area of ​​the graphite material is 0.6 m 2 / g-1.5m 2 / g, optional 0.8m 2 / g-1.4m 2 / g.

[0023] Graphite materials with a specific surface area within the above range can reduce the consumption of active ions in the formation of the solid electrolyte membrane (SEI membrane), reduce the probability of side reactions, and improve the cycle stability of the battery.

[0024] In any embodiment of the present disclosure, the oil absorption value of the graphite material is less than or equal to 45 ml / 100 g, and can be optionally 25 ml / 100 g-44 ml / 100 g.

[0025] The oil absorption value of graphite materials can, to a certain extent, reflect the dispersion of graphite materials in negative electrode slurries. A high oil absorption value of graphite materials means that the graphite materials need to be infiltrated with more dispersants, such as sodium carboxymethyl cellulose, during the dispersion process. The slurry is more likely to settle, resulting in unstable quality during the electrode coating process and uneven electrode thickness after cold pressing. Graphite materials with oil absorption values ​​within an appropriate range have excellent dispersibility in negative electrode slurries. Negative electrode slurries exhibit good anti-settling and uniformity, making it easier to prepare electrodes of uniform quality, reducing the probability of local current density unevenness during charge and discharge, reducing lithium plating caused by electrode polarization, improving battery cycle stability, and extending battery life.

[0026] In any embodiment of the present disclosure, the graphite material has a degree of graphitization of 88%-93%, optionally 89%-92%.

[0027] Graphite materials with a degree of graphitization within the above range have a larger interlayer spacing, which is conducive to the rapid deintercalation of active ions, reduces the expansion rate of the graphite material during the cycle process, and further improves the long cycle life of the battery.

[0028] In any embodiment of the present disclosure, the compaction density of the negative electrode film layer is 1.40 g / cm 3 -1.55g / cm 3 ; and / or, the surface density of the negative electrode film is 7mg / cm 2 ~14mg / cm 2 .

[0029] In any embodiment of the present disclosure, the graphite material is artificial graphite.

[0030] In any embodiment of the present disclosure, the conductive agent includes at least one of conductive carbon black, acetylene black, graphite, graphene, carbon nanotubes, and carbon nanofibers.

[0031] A second aspect of the present disclosure provides an electric device including the secondary battery according to the first aspect of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] To more clearly illustrate the technical solutions of the embodiments of the present disclosure, the following briefly introduces the drawings required for use in the embodiments of the present disclosure. Obviously, the drawings described below are only some embodiments of the present disclosure, and those skilled in the art can derive other drawings based on the drawings without inventive effort.

[0033] FIG1 is a schematic diagram of one embodiment of a secondary battery of the present disclosure.

[0034] FIG. 2 is an exploded schematic diagram of an embodiment of a secondary battery of the present disclosure.

[0035] FIG3 is a schematic diagram of an embodiment of a battery module of the present disclosure.

[0036] FIG. 4 is a schematic diagram of an embodiment of a battery pack according to the present disclosure.

[0037] FIG. 5 is an exploded schematic diagram of the embodiment of the battery pack shown in FIG. 4 .

[0038] FIG6 is a schematic diagram of an embodiment of an electric device including the secondary battery of the present disclosure as a power source.

[0039] In the accompanying drawings, which are not necessarily drawn to scale, reference numerals are as follows: 1 battery pack, 2 upper housing, 3 lower housing, 4 battery module, 5 secondary battery, 51 housing, 52 electrode assembly, 53 cover plate. DETAILED DESCRIPTION

[0040] Below, the embodiments of the secondary battery and the electrical device disclosed in the present invention are described in detail with appropriate reference to the accompanying drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there may be cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present disclosure and are not intended to limit the subject matter described in the claims.

[0041] " scope " disclosed in the present disclosure is limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and selected lower limit and upper limit define the boundary of special scope.The scope that this mode limits can be to include end value or not include end value, and can be combined arbitrarily, and promptly any lower limit can form a scope with any upper limit combination.For example, if the scope of 60-120 and 80-110 is listed for specific parameter, it is understood that the scope of 60-110 and 80-120 is also expected.In addition, if the minimum range value 1 and 2 listed, and if the maximum range value 3,4 and 5 listed, then the following scope can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5.In the present disclosure, unless otherwise specified, numerical range " ab " represents the abbreviation of any real number combination between a and b, and wherein a and b are all real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0042] Unless otherwise specified, all embodiments and optional embodiments of the present disclosure can be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure content of the present disclosure.

[0043] Unless otherwise specified, all technical features and optional technical features of the present disclosure can be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure content of the present disclosure.

[0044] Unless otherwise specified, all steps of the present disclosure may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), which means that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0045] Unless otherwise specified, the terms "include" and "comprising" mentioned in this disclosure may be open-ended or closed-ended. For example, the terms "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0046] Unless otherwise specified, the term "or" is inclusive in this disclosure. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied if any of the following conditions are met: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0047] Unless otherwise specified, the terms used in the present disclosure have the common meanings that are generally understood by those skilled in the art.

[0048] Unless otherwise specified, the numerical values ​​of the parameters mentioned in the present disclosure can be measured using various test methods commonly used in the art, for example, they can be measured according to the test methods given in the present disclosure.

[0049] Unless otherwise specified, in the present disclosure, the term "active ions" refers to ions that can be intercalated and extracted back and forth between the positive electrode and the negative electrode of a secondary battery, including but not limited to lithium ions.

[0050] In the present disclosure, the terms "plurality" and "multiplicity" refer to two or more.

[0051] With the continuous development of the new energy industry, lithium battery energy storage systems have garnered increasing attention. A lithium battery energy storage system is a device that uses lithium batteries for energy storage. It can store energy generated by renewable energy sources such as solar photovoltaics and wind power, and can also serve as an auxiliary facility for the power grid. Compared to power batteries, energy storage batteries place greater emphasis on cycle life. To achieve a long cycle life, researchers typically choose negative electrode materials with high tap density. This is because high tap density negative electrode materials experience less pressure during the cold pressing process of the electrode sheet, which helps maintain particle integrity and reduce internal stress. This reduces the probability of side reactions while maintaining the long-period pore structure of the electrode sheet, ensuring unobstructed lithium insertion pathways and reducing polarization, thereby improving the battery's kinetic performance over long cycles and improving its cycle life. However, during the research and development process, the applicant discovered that after long cycles, as the charge level deepened, severe black spots were prone to form at the negative electrode interface. This caused local current unevenness and polarization, resulting in a loss of electrochemical activity and accelerated capacity decay, leading to a "dive" in battery performance and preventing the full utilization of the high cycle life advantage of high tap density graphite materials.

[0052] Based on this, the present disclosure provides a secondary battery, comprising a negative electrode plate, the negative electrode plate comprising a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector, the negative electrode film layer comprising a negative electrode active material and a conductive agent, the negative electrode active material comprising a graphite material, and the tap density of the graphite material being greater than or equal to 1.2 g / cm 3 ; and based on the total mass of the negative electrode film layer, the mass proportion of the conductive agent is greater than or equal to 1.6%.

[0053] As used herein, the term "tap density" refers to the mass per unit volume of powder in a container measured after being tapped under specified conditions.

[0054] In the present disclosure, the tap density of graphite materials can be tested using methods known in the art. For example, GB / T 5162-2006 can be used for the tap density test using a powder tap density tester. A Dandong Better BT-301 tester can be used, with the following test parameters: a vibration frequency of 250 ± 15 times / minute, an amplitude of 3 ± 0.2 mm, a vibration count of 5000, and a 25 mL graduated cylinder.

[0055] In some embodiments, the tap density of the graphite material is 1.2 g / cm 3 , 1.25g / cm 3 , 1.3g / cm 3 , 1.35g / cm 3 , 1.4g / cm 3 , 1.45g / cm 3 , 1.5g / cm3 or any range of values ​​between them.

[0056] In some embodiments, based on the total mass of the negative electrode film layer, the mass proportion of the conductive agent is 1.6%, 1.7%, 1.8%, 1.9%, 2%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, 3.0% or any numerical range therebetween.

[0057] During the formation phase of a secondary battery, an electrochemical reaction occurs between the negative electrode active material and the electrolyte, forming a solid electrolyte interface (SEI) and accompanied by gas generation. During this process, some negative electrode active material particles have poor electrical contact with surrounding particles, preventing a complete SEI film from forming in time. This results in a delay in SEI film formation and gas production. The gas generated by this delay cannot be discharged in time during the formation process and is retained between the negative electrode plate and the separator. This hinders the charging process in which active ions escape from the positive electrode, pass through the separator, and embed themselves into the negative electrode, potentially causing black spot defects on the negative electrode plate.

[0058] The researchers found that the reason why serious black spots appeared on the negative electrode was that the graphite material with high tap density was easy to form a tight stack, and the electrode could achieve the expected compaction density under a lower cold pressing pressure; and the lower cold pressing pressure made it impossible for the negative electrode material and the conductive agent to form a close electrical contact during the compaction process, which increased the probability of black spots.

[0059] By including a relatively high content of conductive agent in the negative electrode film layer containing high tap density, the electrical contact between graphite materials can be effectively enhanced, the degree of black spots generated in the negative electrode can be reduced, the probability of black spots generated in the negative electrode can be reduced, the advantage of high cycle life of high tap density graphite materials can be fully utilized, and the cycle stability of the battery can be improved.

[0060] In some embodiments, based on the total mass of the negative electrode film layer, the mass proportion of the conductive agent is 1.6%-2.5%, and optionally 1.65%-2.0%.

[0061] Conductive agents within a suitable range can enhance the electrical contact between graphite materials while enabling the slurry to be effectively dispersed. Excessive loss of lithium ions during storage due to excessive amounts of highly active conductive agents in the slurry will not occur, thereby reducing the probability of side reactions occurring during the cycle and further improving the cycle stability, storage performance, and especially high-temperature storage performance of the secondary battery.

[0062] In some embodiments, the tap density of the graphite material is 1.20 g / cm 3 -1.42g / cm 3 In some embodiments, the tap density of the graphite material is 1.25 g / cm 3-1.35g / cm 3 .

[0063] A tap density within an appropriate range can improve the cycle stability of the secondary battery while reducing the probability of black spots on the negative electrode sheet and improving the quality of the negative electrode sheet.

[0064] In some embodiments, the graphite material includes primary particles; in some embodiments, based on the total number of particles of the graphite material, the primary particles in the graphite material account for greater than or equal to 85%.

[0065] As used herein, the term "primary particles" refers to particles in a non-agglomerated state.

[0066] In the present disclosure, the proportion of primary particles in the graphite material can be tested by methods known in the art. As an example, a test sample is randomly selected in the negative electrode film layer, and multiple test areas are randomly selected in the test sample. Images of the multiple test areas are obtained using a scanning electron microscope. The proportion of the number of graphite material particles with primary particle morphology in each image to the total number of graphite material particles is counted, and the average value of the multiple statistical results is the proportion of primary particles in the graphite material.

[0067] In some embodiments, based on the total number of particles of the graphite material, the number of primary particles in the graphite material accounts for 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, 100% or any range therebetween.

[0068] Compared with agglomerated secondary particles, primary particles can effectively reduce the grain boundary content, reduce the probability of side reactions between graphite materials and electrolytes, improve the uniformity of the electrodes, and further improve the cycle stability of secondary batteries.

[0069] In some embodiments, the graphite material has a Dv1 of 1.0 μm to 4.0 μm. In some embodiments, the graphite material has a Dv1 of 1.5 μm to 3.0 μm.

[0070] The volume distribution particle size Dv1 of the graphite negative electrode active material represents the particle size corresponding to when the cumulative volume distribution percentage of the material reaches 1%.

[0071] In the present disclosure, the volume distribution particle size Dv1 of the graphite material can be measured using methods known in the art. For example, referring to GB / T 19077-2016, it can be measured using a laser particle size analyzer. The testing instrument can be a Mastersizer 3000 laser particle size analyzer manufactured by Malvern Instruments Ltd., UK.

[0072] In some embodiments, the Dv1 of the graphite material is 1.0 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, 2.0 μm, 2.1 μm, 2.2 μm, 2.3 μm, 2.4 μm, 2.5 μm, 2.6 μm, 2.7 μm, 2.8 μm, 2.9 μm, 3.0 μm, 3.5 μm, 4.0 μm or any range therebetween.

[0073] Applicants have discovered that the Dv1 particle size of graphite materials has a significant impact on the electrochemical performance of secondary batteries. A graphite material with a Dv1 within an appropriate range not only allows the negative electrode film layer to contain a certain proportion of fine powder, namely, small-particle graphite material, which is beneficial for improving the electrical contact of the negative electrode film layer, improving dynamic performance, reducing the severity of negative electrode black spots during battery cycling, and reducing the probability of battery electrochemical performance "diving." It also reduces the probability of small-particle graphite material clogging the pores of the negative electrode film layer, maintaining normal electrode pores, allowing for high electrolyte wettability, reducing the risk of lithium plating at the negative electrode, and further improving the battery's cycling stability.

[0074] In some embodiments, the graphite material has a Dv50 of 10 μm to 18 μm. In some embodiments, the graphite material has a Dv50 of 12 μm to 16 μm.

[0075] In some embodiments, the Dv50 of the graphite material is 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, or any range therebetween.

[0076] Graphite materials with Dv50 within the above range can easily form effective connections in the negative electrode film layer to improve the conductivity of active ions and electrons; they can also form a reasonable pore structure in the negative electrode film layer, thereby comprehensively improving the cycle stability of the secondary battery.

[0077] In some embodiments, the particle size distribution of the graphite material (Dv90-Dv10) / Dv50 is 1.20-1.70. In some embodiments, the particle size distribution of the graphite material (Dv90-Dv10) / Dv50 is 1.35-1.60.

[0078] As used herein, the term "particle size distribution" refers to a measure of the breadth of the distribution of particle sizes.

[0079] In some embodiments, the particle size distribution of the graphite material (Dv90-Dv10) / Dv50 is 1.20, 1.25, 1.30, 1.35, 1.40, 1.45, 1.50, 1.55, 1.60, 1.65, 1.70 or any range therebetween.

[0080] The particle size distribution of the graphite material is within the above range, which is beneficial to reducing the bridging phenomenon between the graphite materials and improving the electrical contact of the negative electrode film layer, and is also beneficial to improving the uniformity of the dispersion of the graphite material in the slurry, thereby comprehensively improving the cycle performance of the battery.

[0081] In some embodiments, the specific surface area of ​​the graphite material is 0.6 m 2 / g-1.5m 2 In some embodiments, the specific surface area of ​​the graphite material is 0.8 m 2 / g-1.4m 2 / g.

[0082] In the present disclosure, the specific surface area of ​​a graphite material can be measured using methods known in the art. For example, the specific surface area of ​​a graphite material can be measured using nitrogen adsorption specific surface area analysis according to GB / T 19587-2017 and calculated using the BET (Brunauer Emmett Teller) method. The testing instrument can be a Micromeritics Tri-Star 3020 specific surface area pore size analyzer.

[0083] In some embodiments, the specific surface area of ​​the graphite material is 0.6 m 2 / g, 0.7m 2 / g, 0.8m 2 / g, 0.9m 2 / g、1m 2 / g, 1.1m 2 / g, 1.2m 2 / g, 1.3m 2 / g, 1.4m 2 / g, 1.5m 2 / g or any range of values ​​between them.

[0084] Graphite materials with a specific surface area within the above range can reduce the consumption of active ions in the formation of the solid electrolyte membrane (SEI membrane), reduce the probability of side reactions, and improve the cycle stability of the battery.

[0085] In some embodiments, the graphite material has an oil absorption value of less than or equal to 45 ml / 100 g. In some embodiments, the graphite material has an oil absorption value of 25 ml / 100 g to 44 ml / 100 g.

[0086] As used herein, the term "oil absorption value" refers to the volume of linseed oil that can be absorbed by 100 g of graphite material. For example, an oil absorption value of 40 ml / 100 g of graphite material means that 100 g of graphite material can absorb 40 ml of linseed oil.

[0087] The oil absorption of graphite materials can be tested using methods and equipment known in the art. For example, the following steps are performed: Test oil and graphite material samples are obtained, and a torque threshold is set for the oil absorption tester. Oil is then added to the sample in the mixing chamber of the oil absorption tester at a constant rate. As the amount of oil absorbed by the sample increases, the viscosity of the sample-oil mixture increases. When the viscosity of the mixture reaches the preset torque threshold, the tester stops and calculates the volume of oil absorbed per unit mass of the sample. This value is the oil absorption value (QI) of the sample. The test oil used is linseed oil (DBP), and the torque threshold is 1N.

[0088] In some embodiments, the oil absorption value of the graphite material may be 35 ml / 100 g, 36 ml / 100 g, 37 ml / 100 g, 38 ml / 100 g, 39 ml / 100 g, 40 ml / 100 g, 41 ml / 100 g, 42 ml / 100 g, 43 ml / 100 g, 44 ml / 100 g, 45 ml / 100 g or any range therebetween.

[0089] The oil absorption value of graphite materials can, to a certain extent, reflect the dispersion of graphite materials in negative electrode slurries. A high oil absorption value of graphite materials means that the graphite materials need to be infiltrated with more dispersants, such as sodium carboxymethyl cellulose, during the dispersion process. The slurry is more likely to settle, resulting in unstable quality during the electrode coating process and uneven electrode thickness after cold pressing. Graphite materials with oil absorption values ​​within an appropriate range have excellent dispersibility in negative electrode slurries. Negative electrode slurries exhibit good anti-settling and uniformity, making it easier to prepare electrodes of uniform quality, reducing the probability of local current density unevenness during charge and discharge, reducing lithium plating caused by electrode polarization, improving battery cycle stability, and extending battery life.

[0090] In some embodiments, the graphite material has a degree of graphitization of 88% to 93%. In some embodiments, the graphite material has a degree of graphitization of 89% to 92%.

[0091] As used herein, the term "degree of graphitization" refers to an indicator measuring the degree to which carbon atoms form a close-packed hexagonal graphite crystal structure.

[0092] In the present disclosure, the graphitization degree of the graphite material can be tested by methods known in the art. As an example, an X-ray diffractometer (such as Bruker D8 Discover) is used for testing, and the average interlayer spacing d of the (002) crystal plane in the crystal structure of the graphite material is obtained by referring to JIS K0131-1996 and JB / T4220-2011. 002 Then according to the formula g=(0.344-d 002) / (0.344-0.3354)×100% to calculate the degree of graphitization. 002 It is the average interlayer spacing of the (002) crystal planes in the crystal structure of graphite material expressed in nanometers (nm).

[0093] In some embodiments, the graphite material has a degree of graphitization of 88%, 89%, 90%, 91%, 92%, 93%, or any range therebetween.

[0094] Graphite materials with a degree of graphitization within the above range have a larger interlayer spacing, which is conducive to the rapid deintercalation of active ions, reduces the expansion rate of the graphite material during the cycle process, and further improves the long cycle life of the battery.

[0095] In some embodiments, the compaction density of the negative electrode film layer is 1.40 g / cm 3 -1.55g / cm 3 ; and / or, the surface density of the negative electrode film layer is 7mg / cm 2 ~14mg / cm 2 .

[0096] In the present disclosure, the compaction density of the negative electrode film layer can be tested using methods known in the art. As an example, an electronic balance is used to weigh a negative electrode sheet test sample with an area of ​​S, and the weight is recorded as W1, and a caliper is used to measure the thickness T1 of the negative electrode sheet. Then, the negative electrode film of the weighed negative electrode sheet is wiped off, the weight of the negative electrode current collector is weighed, recorded as W2, and the thickness T2 of the negative electrode current collector is measured using a caliper. Then the compaction density of the negative electrode film layer PD = (W1-W2) / [(T1-T2)×S]

[0097] In some embodiments, the compaction density of the negative electrode film layer is 1.40 g / cm 3 , 1.43g / cm 3 , 1.46g / cm 3 , 1.49g / cm 3 , 1.52g / cm 3 , 1.55g / cm 3 or any range of values ​​between them.

[0098] In the present disclosure, the areal density of the negative electrode film layer can be measured using methods known in the art. As an example, a cold-pressed negative electrode sheet is punched into small discs with an area of ​​S1. The discs are weighed and recorded as M1. The negative electrode film from the weighed negative electrode sheet is then wiped off, and the weight of the negative electrode current collector is weighed and recorded as M0. The areal density of the negative electrode film = (weight of the negative electrode sheet M1 - weight of the negative electrode current collector M0) / S1.

[0099] In some embodiments, the surface density of the negative electrode film layer is 7 mg / cm 2 , 8mg / cm 2 , 9mg / cm 2 、10mg / cm 2 , 11mg / cm 2 , 12mg / cm 2 、13mg / cm 2 、14mg / cm 2 or any range of values ​​between them.

[0100] In some embodiments, the graphite material is artificial graphite.

[0101] In some embodiments, the conductive agent includes at least one of conductive carbon black, acetylene black, graphite, graphene, carbon nanotubes, and carbon nanofibers.

[0102] In some embodiments, the method for preparing the graphite material includes the following steps: providing raw materials, wherein the raw materials include at least one of petroleum coke, needle coke, and asphalt coke, and based on the total volume of the raw material structure, the volume of the mosaic-type organizational structure and the regional-type organizational structure in the raw material accounts for not less than 60%; processing the raw materials to obtain a precursor; and graphitizing the precursor to obtain the graphite material.

[0103] In this article, the term "petroleum coke" refers to the coke formed by high-temperature carbonization of petroleum residue or petroleum asphalt, which is called petroleum coke.

[0104] In this article, the term "needle coke" refers to coal tar pitch or petroleum pitch, which undergoes liquid phase carbonization to generate anisotropic mesophase, and then undergoes high-temperature carbonization and other processes to generate coke with a needle-like texture, which is called needle coke.

[0105] As used herein, the term "pitch coke" refers to the solid material produced by carbonizing coal tar pitch at high temperatures.

[0106] The above-mentioned raw materials generally include at least one of mosaic, regional, and fibrous structures. Generally, based on the morphological characteristics and isochromatic zone size of the char material under a polarizing microscope, isochromatic zones with a size less than 30 μm are classified as mosaic; isochromatic zones with a size greater than 30 μm are classified as regional; and anisotropic banded isochromatic zones are classified as fibrous.

[0107] In the present disclosure, the proportion of mosaic structure and regional structure in the raw material can be tested by methods known in the art. As an example, according to the provisions of GB 1997-89, the raw material is crushed to 1mm and mixed, and 40g to 50g is separated. A square hole sieve is used to take 4g to 5g of a 0.07mm to 1.0mm grade sample for film making; according to the provisions of MT 116.1-86, powder coke and block coke optical films are prepared. The diameter of the powder coke optical film shall not be less than 22mm, and the volume occupied by the cement shall be less than 1 / 3; the sample is placed on a slide with clay, flattened, and then placed on the stage for focusing. After calibrating the microscope, the polarizer and analyzer are adjusted to make them orthogonal. Insert the azurite inspection plate (1λ) so that the field of view shows the interference color of the first-order red; determine the step length of the moving ruler to ensure that more than 400 effective measuring points are evenly distributed, with a point spacing of 0.3mm to 0.5mm and a line spacing of 0.5mm to 0.8mm. Starting from one end of the sample, determine the microstructure category under the intersection of the crosshairs, and divide the number of effective measuring points of the mosaic structure and regional structure optical organization by the total number of statistical test points as the volume content of the mosaic structure and regional structure in the raw material.

[0108] In some embodiments, based on the total volume of the raw material structure, the volume proportion of the mosaic structure and the regional structure in the raw material is 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 100% or any numerical range therebetween.

[0109] In this article, the term "graphitization treatment" refers to the heat treatment process of graphite materials. Under the action of high temperature, the graphite material transforms from a two-dimensional carbon network structure to a three-dimensional ordered structure through "microcrystal" growth.

[0110] In some embodiments, the power of the graphitization process is 70% to 90% of the rated power of the equipment.

[0111] In some embodiments, the power of the graphitization process can be selected to be 70%, 75%, 80%, 85%, 90% of the rated power of the equipment, or any range therebetween. It is understood that graphitization processing equipment refers to any device capable of performing graphitization processing, including but not limited to Acheson furnaces, box furnaces, internal furnaces, continuous graphitization furnaces, electric calcining furnaces, medium-frequency furnaces, tubular furnaces, and the like. The rated power of graphitization processing equipment produced by different manufacturers may vary, and the rated power can be selected based on actual conditions.

[0112] In some embodiments, the graphitization treatment equipment is an inner string furnace, and the rated power of the inner string furnace is 25000-32000W.

[0113] In some embodiments, the graphitization treatment equipment is an Acheson furnace, and the rated power of the Acheson furnace is 28,000-30,000W.

[0114] In some embodiments, the maximum power of the graphitization process is 23,000 W to 25,000 W. For example, it can be 23,000 W, 23,500 W, 24,000 W, 24,500 W, 25,000 W, or any range therebetween.

[0115] In some embodiments, the graphitization treatment is performed at a constant power time of maximum power for 10 hours to 50 hours.

[0116] In some embodiments, the graphitization treatment is performed at a constant power time of maximum power of 10 h, 13 h, 16 h, 19 h, 22 h, 25 h, 28 h, 31 h, 33 h, 36 h, 39 h, 42 h, 45 h, 48 h, 50 h, or any range therebetween.

[0117] In some embodiments, the graphitization treatment equipment is an internal series furnace, and the graphitization treatment is performed at a constant power time of maximum power for 10 hours to 30 hours.

[0118] In some embodiments, the graphitization treatment equipment is an Acheson furnace, and the graphitization treatment is performed at a constant power time of maximum power for 30 hours to 50 hours.

[0119] In some embodiments, the temperature of the graphitization treatment is 2800°C to 3000°C.

[0120] In some embodiments, the temperature of the graphitization treatment is 2800° C., 2900° C., 3000° C., or any range therebetween.

[0121] By controlling the proportion of the area of ​​the embedded and regional structures in the raw materials within an appropriate range, on the one hand, graphite materials with low graphitization degree and low powder compaction density can be obtained, which is beneficial to improving the cycle performance of the battery. On the other hand, graphite materials with high gram capacity and tap density can also be obtained, thereby improving the energy density of the battery.

[0122] In some embodiments, the processing of raw materials to obtain a precursor specifically includes the following steps: crushing, shaping and grading the raw materials to obtain secondary raw materials; removing part of the fine powder in the secondary raw materials, where the removed fine powder accounts for 18%-45% of the total mass of the secondary raw materials, to obtain a precursor, wherein the Dv50 of the fine powder is 3μm-7μm, and Dv99 is less than or equal to 30μm.

[0123] Crushing is the process of reducing the particle size of the raw material. The raw material can be crushed by any mechanical device such as a crusher, a mechanical mill, etc. In some embodiments, the raw material is crushed to a predetermined particle size before being screened.

[0124] The shaping treatment can reduce the burrs on the surface of the crushed raw materials, which is conducive to obtaining graphite materials with high sphericity.

[0125] Classification is the process of adjusting the particle size distribution of the raw materials to obtain a first precursor that meets the required particle size. The particle size and distribution of the precursor can be controlled by adjusting the classification frequency and air flow rate. Classification can reduce the content of large and small particles in the precursor.

[0126] In some embodiments, removing some fine powder in the secondary raw material to regulate the Dv1 of the graphite material is beneficial for making the Dv1 of the graphite material within a suitable range, so that the graphite material can easily form good electrical contact in the negative electrode film layer.

[0127] In some embodiments, the volume distribution particle size Dv50 of the precursor is 12 μm-18 μm.

[0128] In some embodiments, the volume distribution particle size Dv50 of the precursor can be selected as 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm or any numerical range therebetween.

[0129] In some embodiments, the particle size distribution of the precursor is (Dv90-Dv10) / Dv50 is 1.3-2.0.

[0130] In some embodiments, the particle size distribution of the precursor (Dv90-Dv10) / Dv50 can be selected as 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9 or 2.0.

[0131] Controlling the volume distribution particle size Dv50 or (Dv90-Dv10) / Dv50 of the precursor within a suitable range helps to prepare graphite materials with a volume distribution particle size Dv50 or (Dv90-Dv10) / Dv50 within a suitable range, which can improve the cycle performance of the battery.

[0132] The present disclosure has no particular restrictions on the types of secondary batteries. For example, the secondary battery can be a lithium-ion battery, etc. In general, a secondary battery includes a positive electrode plate, a negative electrode plate, and an electrolyte, etc. During the charge and discharge process of the secondary battery, active ions are embedded and extracted back and forth between the positive electrode plate and the negative electrode plate, and the electrolyte plays a role in conducting active ions between the positive electrode plate and the negative electrode plate. The present disclosure has no particular restrictions on the type of the electrolyte, and it can be selected according to actual needs. For example, the electrolyte can be selected from at least one of a solid electrolyte and a liquid electrolyte (i.e., an electrolyte). Secondary batteries using electrolytes and some secondary batteries using solid electrolytes can also include an isolation membrane, which is arranged between the positive electrode plate and the negative electrode plate to play an isolation role.

[0133] [Negative electrode]

[0134] In some embodiments, the negative electrode plate includes a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector. For example, the negative electrode current collector has two surfaces that are opposite to each other in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposing surfaces of the negative electrode current collector.

[0135] In some embodiments, the negative electrode film layer may further include other negative electrode active materials in addition to the above-mentioned graphite materials. In some embodiments, the other negative electrode active materials include, but are not limited to, one or more of conventional natural graphite, artificial graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. The silicon-based materials may include one or more of elemental silicon, silicon oxide, silicon-carbon composite, silicon-nitrogen composite, and silicon alloy materials. The tin-based materials may include one or more of elemental tin, tin oxide, and tin alloy materials.

[0136] In some embodiments, the negative electrode film layer may further optionally include a negative electrode binder. The present disclosure does not particularly limit the type of the negative electrode binder. As examples, the negative electrode binder may include one or more of styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, water-based acrylic resin (e.g., polyacrylic acid PAA, polymethacrylic acid PMAA, sodium polyacrylate PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethyl chitosan (CMCS).

[0137] In some embodiments, the negative electrode film layer may further include other additives. For example, the other additives may include a thickener, such as sodium carboxymethyl cellulose (CMC), a PTC thermistor material, and the like.

[0138] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. As an example of a metal foil, copper foil may be used. The composite current collector may include a polymer material base layer and a metal material layer formed on at least one surface of the polymer material base layer. As an example, the metal material may include one or more of copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy. As an example, the polymer material base layer may include one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS) and polyethylene (PE).

[0139] The negative electrode film layer is typically formed by coating a negative electrode slurry onto a negative electrode current collector, drying, and cold pressing. The negative electrode slurry is typically formed by dispersing the negative electrode active material, an optional conductive agent, an optional binder, and other optional additives in a solvent and stirring until uniformly mixed. The solvent may be, but is not limited to, N-methylpyrrolidone (NMP) or deionized water.

[0140] The negative electrode plate does not exclude other additional functional layers in addition to the negative electrode film layer. For example, in some embodiments, the negative electrode plate described in the present disclosure further includes a conductive primer layer (e.g., composed of a conductive agent and a binder) sandwiched between the negative electrode current collector and the negative electrode film layer and disposed on the surface of the negative electrode current collector; in some embodiments, the negative electrode plate described in the present disclosure further includes a protective layer covering the surface of the negative electrode film layer.

[0141] [Positive electrode]

[0142] In some embodiments, the positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector. For example, the positive electrode current collector has two surfaces that are opposite to each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposing surfaces of the positive electrode current collector.

[0143] The positive electrode current collector may be a metal foil or a composite current collector. As an example of a metal foil, aluminum foil may be used. The composite current collector may include a polymer material base layer and a metal material layer formed on at least one surface of the polymer material base layer. As an example, the metal material may include one or more of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy. As an example, the polymer material base layer may include one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS) and polyethylene (PE).

[0144] The positive electrode film layer generally includes a positive electrode active material, an optional binder, and an optional conductive agent. The positive electrode film layer is generally formed by coating a positive electrode slurry on the positive electrode current collector and then drying and cold pressing. The positive electrode slurry is generally formed by dispersing a positive electrode active material, an optional conductive agent, an optional binder, and any other components in a solvent and stirring evenly. The solvent can be N-methylpyrrolidone (NMP), but is not limited thereto. As an example, the binder used for the positive electrode film layer may include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin. As an example, the conductive agent used for the positive electrode film layer includes one or more of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0145] The positive electrode active material can be a positive electrode active material for secondary batteries well-known in the art.

[0146] When the secondary battery of the present disclosure is a lithium-ion battery, the positive electrode active material may include, but is not limited to, one or more of lithium-containing transition metal oxides, lithium-containing phosphates, and their respective modified compounds. Examples of the lithium transition metal oxides may include, but are not limited to, one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and their respective modified compounds. Examples of the lithium-containing phosphates may include, but are not limited to, lithium iron phosphate, a composite material of lithium iron phosphate and carbon, lithium manganese phosphate, a composite material of lithium manganese phosphate and carbon, lithium manganese iron phosphate, a composite material of lithium manganese iron phosphate and carbon, and their respective modified compounds.

[0147] In some embodiments, in order to further improve the energy density of the secondary battery, the positive electrode active material for the lithium-ion battery may include a lithium transition metal oxide having the general formula Li a Ni b Co c M d O e A f and its modified compounds, where 0.8 ≤ a ≤ 1.2, 0.5 ≤ b < 1, 0 < c < 1, 0 < d < 1, 1 ≤ e ≤ 2, 0 ≤ f ≤ 1, M is selected from one or more of Mn, Al, Zr, Zn, Cu, Cr, Mg, Fe, V, Ti, and B, and A is selected from one or more of N, F, S, and Cl.

[0148] In some embodiments, as examples, the positive electrode active material for lithium ion batteries may include LiCoO2, LiNiO2, LiMnO2, LiMn2O4, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2(NCM333),LiNi 0.5 Co 0.2 Mn 0.3 O2(NCM523), LiNi 0.6 Co 0.2 Mn 0.2 O2(NCM622),LiNi 0.8 Co 0.1 Mn 0.1 O2(NCM811), LiNi 0.85 Co 0.15 Al 0.05 One or more of O2, LiFePO4 and LiMnPO4.

[0149] In the present disclosure, the modified compounds of the above-mentioned positive electrode active materials may be the ones subjected to doping modification and / or surface coating modification on the positive electrode active materials.

[0150] [Electrolytes]

[0151] In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0152] The type of the electrolyte salt is not particularly limited and can be selected according to actual needs.

[0153] When the secondary battery of the present disclosure is a lithium ion battery, as an example, the electrolyte salt may include one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bisfluorosulfonyl imide (LiFSI), lithium bistrifluoromethanesulfonyl imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalatoborate (LiDFOB), lithium dioxalatoborate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorobisoxalatophosphate (LiDFOP) and lithium tetrafluorooxalatophosphate (LiTFOP).

[0154] The type of the solvent is not specifically limited and can be selected according to actual needs. In some embodiments, for example, the solvent may include ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), ethyl methyl sulfone (EMS) and diethyl sulfone (ESE). One or more.

[0155] In some embodiments, the electrolyte may further optionally include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, or additives capable of improving certain properties of the secondary battery, such as additives that improve the overcharge performance of the secondary battery, additives that improve the high-temperature performance of the secondary battery, and additives that improve the low-temperature power performance of the secondary battery.

[0156] [Isolation film]

[0157] The present disclosure has no particular limitation on the type of the isolation membrane, and any known isolation membrane with a porous structure having good chemical stability and mechanical stability can be selected.

[0158] In some embodiments, the material of the isolation membrane may include one or more of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The isolation membrane may be a single-layer film or a multi-layer composite film. When the isolation membrane is a multi-layer composite film, the materials of each layer may be the same or different.

[0159] In some embodiments, the positive electrode sheet, the separator, and the negative electrode sheet can be formed into an electrode assembly through a winding process or a lamination process.

[0160] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0161] In some embodiments, the outer packaging can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging can also be a soft package, such as a bag-type soft package. The material of the soft package can be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0162] The present disclosure has no particular limitation on the shape of the secondary battery, which can be cylindrical, square, or any other shape. FIG1 shows a secondary battery 5 with a square structure as an example.

[0163] In some embodiments, as shown in FIG2 , the outer packaging may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 is used to cover the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation film can be formed into an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is impregnated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, which can be adjusted according to demand.

[0164] The preparation method of the secondary battery disclosed herein is well known. In some embodiments, a positive electrode sheet, a separator, a negative electrode sheet, and an electrolyte can be assembled to form a secondary battery. As an example, the positive electrode sheet, separator, and negative electrode sheet can be wound or laminated to form an electrode assembly. The electrode assembly is then placed in an outer packaging, dried, and then injected with electrolyte. The secondary battery is then vacuum packaged, allowed to stand, formed, and shaped to obtain the secondary battery.

[0165] In some embodiments of the present disclosure, the secondary batteries according to the present disclosure may be assembled into a battery module. The battery module may contain multiple secondary batteries, and the specific number may be adjusted according to the application and capacity of the battery module.

[0166] Figure 3 is a schematic diagram of an exemplary battery module 4. As shown in Figure 3 , within the battery module 4, multiple secondary batteries 5 may be arranged sequentially along the length of the battery module 4. Of course, any other arrangement is also possible. Furthermore, the multiple secondary batteries 5 may be secured using fasteners.

[0167] Optionally, the battery module 4 may further include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0168] In some embodiments, the battery modules described above may also be assembled into a battery pack, and the number of battery modules contained in the battery pack may be adjusted according to the application and capacity of the battery pack.

[0169] Figures 4 and 5 are schematic diagrams of an exemplary battery pack 1. As shown in Figures 4 and 5, the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box comprises an upper case 2 and a lower case 3. The upper case 2 covers the lower case 3 and forms an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0170] Electrical devices

[0171] The present disclosure also provides an electric device, which includes at least one of the secondary battery, battery module, or battery pack of the present disclosure. The secondary battery, battery module, or battery pack can be used as a power source for the electric device, and can also be used as an energy storage unit for the electric device. The electric device can be, but is not limited to, a mobile device (such as a mobile phone, a tablet computer, a laptop computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship, a satellite, an energy storage system, etc.

[0172] The electrical device may select a secondary battery, a battery module or a battery pack according to its usage requirements.

[0173] FIG6 is a schematic diagram of an exemplary electric device. The electric device may be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the electric device, a battery pack or battery module may be used.

[0174] As another example, the electric device may be a mobile phone, a tablet computer, a laptop computer, etc. Such an electric device is usually required to be lightweight and thin, and may use a secondary battery as a power source.

[0175] Example

[0176] The following examples describe the present disclosure in more detail. These examples are intended to be illustrative only, as various modifications and variations within the scope of the present disclosure will be apparent to those skilled in the art. Unless otherwise stated, all parts, percentages, and ratios reported in the following examples are by mass, and all reagents used in the examples are commercially available or synthesized according to conventional methods and can be used directly without further processing, and all instruments used in the examples are commercially available.

[0177] Material 1

[0178] Petroleum coke with a mosaic and regional structure accounting for 63.1% is crushed; the crushed material is shaped to obtain a secondary raw material, and fine powder accounting for 18% by mass of the secondary raw material is removed. The remaining material is called a precursor. The obtained precursor has a Dv50 particle size of 15.5μm and a particle size distribution of (Dv90-Dv10) / Dv50 of 1.97; fine powder refers to a component with a Dv50 of 3μm-7μm and a Dv99 of less than or equal to 30μm.

[0179] The precursor was graphitized in an Acheson furnace (the rated power of the Acheson furnace was 30,000 W) at a temperature of 2,800°C and a maximum power of 24,000 W. After maintaining the maximum power for 40 hours, the surface temperature of the Acheson furnace crucible was cooled to 400°C, the graphite crucible was taken out, and the graphite material was obtained after screening and demagnetization.

[0180] The tap density of graphite material is 1.22g / cm 3 , Dv1 is 1.8μm, Dv50 is 14.6μm, particle size distribution (Dv90-Dv10) / Dv50 is 1.42, and the specific surface area is 1.14m 2 / g, and the degree of graphitization is 91.1%.

[0181] Materials 2-5

[0182] The preparation method of material 2-5 is similar to that of material 1, except that the mass ratio of fine powder removed from the secondary raw material is adjusted. Detailed parameters are shown in Table 1.

[0183] Table 1

[0184] Example 1

[0185] Preparation of secondary batteries

[0186] The prepared material 1, conductive agent carbon black (Super P), binder styrene-butadiene rubber, and thickener sodium carboxymethyl cellulose were thoroughly stirred and mixed in an appropriate amount of deionized water at a weight ratio of 95.4:1.8:1.0:1.8 to form a negative electrode slurry. The negative electrode slurry was coated on both surfaces of the negative electrode current collector copper foil, dried, and cold pressed to obtain a negative electrode sheet. The compacted density of the negative electrode film was 1.48 g / cm 3 , with a surface density of 9.4 mg / cm 2 .

[0187] The positive electrode active material lithium iron phosphate (LFP), the conductive agent Super P, and the binder PVDF were mixed in a mass ratio of 97:1:2, and the solvent N-methylpyrrolidone NMP was added. The mixture was stirred in a vacuum mixer until the system became uniform to obtain a positive electrode slurry. The positive electrode slurry was evenly coated on the positive electrode current collector aluminum foil, and the positive electrode sheets were obtained after drying, cold pressing, and slitting. The compacted density of the positive electrode film layer is 2.50g / cm 3 , with a surface density of 20 mg / cm 2 .

[0188] In an argon atmosphere glove box with a water content of less than 10 ppm, diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and ethylene carbonate (EC) were mixed in a weight ratio of 1:1:1 to obtain an organic solvent. LiPF6 was then dissolved in the organic solvent to prepare an electrolyte with a LiPF6 concentration of 1.0 mol / L. Vinylene carbonate (VC) was then added, and the VC content was 2% of the total mass of the electrolyte.

[0189] Polypropylene film is used as the isolation film.

[0190] The positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator placed between the positive and negative electrode sheets to serve as an isolation, and then wound to obtain an electrode assembly; the electrode assembly is placed in an outer package, dried, and then injected with electrolyte, and after vacuum packaging, standing, forming, shaping and other processes, a lithium-ion battery is obtained.

[0191] Examples 2-7 and Comparative Examples 1-2

[0192] The preparation method of the secondary battery is similar to that of Example 1, except that different graphite materials are used or the content of the conductive agent is adjusted, as shown in Table 2 for details.

[0193] Performance Testing

[0194] (1) Formation-End State of Charge (SOC) Test

[0195] At 25° C., the battery after injection prepared in the example and the comparative example was charged at 0.33C, and the gas generated during the formation process was collected. When the total amount of gas did not increase, the corresponding SOC was recorded.

[0196] (2) Dark spot test

[0197] At 25°C, the batteries of the above embodiments and comparative examples were charged at a constant current of 0.33C to a voltage of 3.65V, disassembled in a drying room, and the surface of the negative electrode plate was observed to see whether there were black spots; the total area of ​​black spots / total area of ​​the negative electrode plate ≤1%, and the area of ​​black spots in a single electrode plate / area of ​​a single electrode plate ≤8% were defined as first-level black spots; 1% <total area of ​​black spots / total area of ​​the negative electrode plate ≤3%, or 8% <area of ​​black spots in a single electrode plate / area of ​​a single electrode plate ≤15% were defined as second-level black spots; the total area of ​​black spots / total area of ​​the negative electrode plate >3%, or the area of ​​black spots in a single electrode plate / area of ​​a single electrode plate >15%, were defined as third-level black spots.

[0198] (3) Cycle performance test of secondary batteries

[0199] At 25°C, the batteries of the above examples and comparative examples were charged at a constant current of 1C to a voltage of 3.65V. They were then charged at a constant voltage of 3.65V to a current of ≤0.05C. The batteries were then discharged at a constant current of 1C to a voltage of 2.5V. This constituted one charge and discharge cycle, and the discharge capacity (C1) of the first cycle was recorded. This charge and discharge cycle was repeated until the battery capacity decayed to 70% of the initial capacity (C1). The number of cycles was recorded.

[0200] Test results

[0201] Batteries of various examples and comparative examples were prepared according to the above methods, and various performance parameters were measured. The results are shown in the table below.

[0202] Table 2

[0203] As shown in Table 2, Examples 1-7, the secondary battery includes a negative electrode plate, the negative electrode plate includes a negative electrode current collector and a negative electrode film layer provided on at least one surface of the negative electrode current collector, the negative electrode film layer includes a negative electrode active material and a conductive agent, the negative electrode active material includes a graphite material, and the tap density of the graphite material is greater than or equal to 1.2 g / cm 3 ; and the negative electrode film layer includes a conductive agent. When the mass proportion of the conductive agent is greater than or equal to 1.6% based on the total mass of the negative electrode film layer, the secondary battery can reduce the severity of black spots formed by high tap density graphite materials at the negative electrode, reduce the probability of battery performance "diving", and improve the battery's cycle stability.

[0204] As can be seen from Examples 1-4, when the Dv1 of the graphite material is 1.5μm-3.0μm, the battery's formation-end SOC is further reduced. Formation is the process of charging the battery for the first time and activating the electrochemical reaction inside the battery. As the formation charge proceeds, lithium ions are deintercalated from the positive electrode material, transported through the diaphragm by the electrolyte to the negative electrode and embedded in the negative electrode material, thereby forming a potential difference between the positive and negative electrodes, converting electrical energy into chemical energy of the battery. When the formation voltage reaches a certain value, an oxidation-reduction reaction occurs at the solid-liquid interface between the negative electrode and the electrolyte, generating a solid electrolyte interface film (SEI film). The higher the formation-end SOC, the higher the amount of electricity charged during the formation process, the more lithium ions consumed to form the SEI film, the lower the battery's first efficiency, and the greater the severity of black spots generated at the negative electrode of the battery. Reducing the battery's formation-end SOC is beneficial to reducing the severity of negative electrode black spots and improving the battery's cycle stability.

[0205] It should be noted that the present disclosure is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and within the scope of the technical solution of the present disclosure, embodiments having substantially the same structure as the technical concept and exerting the same effects are all included in the technical scope of the present disclosure. In addition, within the scope of the present disclosure, various modifications that can be imagined by those skilled in the art to the embodiments, and other methods constructed by combining some of the constituent elements of the embodiments are also included in the scope of the present disclosure.

Claims

1. A secondary battery, characterized in that: The invention comprises a negative electrode plate, wherein the negative electrode plate comprises a negative electrode current collector and a negative electrode film layer arranged on at least one surface of the negative electrode current collector, wherein the negative electrode film layer comprises a negative electrode active material and a conductive agent, wherein the negative electrode active material comprises a graphite material, and the tap density of the graphite material is greater than or equal to 1.2 g / cm 3 ; and based on the total mass of the negative electrode film layer, the mass proportion of the conductive agent is greater than or equal to 1.6%.

2. The secondary battery according to claim 1, characterized in that: Based on the total mass of the negative electrode film layer, the mass proportion of the conductive agent is 1.6%-2.5%, and can be optionally 1.65%-2.0%.

3. The secondary battery according to claim 1 or 2, characterized in that: The tap density of the graphite material is 1.20 g / cm 3 -1.42g / cm 3 , optional 1.25g / cm 3 -1.35g / cm 3 .

4. The secondary battery according to any one of claims 1 to 3, characterized in that: The graphite material includes primary particles; optionally, based on the total number of particles of the graphite material, the number of primary particles in the graphite material accounts for greater than or equal to 85%.

5. The secondary battery according to any one of claims 1 to 4, characterized in that: The Dv1 of the graphite material is 1.0 μm-4.0 μm, and can be optionally 1.5 μm-3.0 μm.

6. The secondary battery according to any one of claims 1 to 5, characterized in that: The volume distribution particle size Dv50 of the graphite material is 10 μm-18 μm, and can be optionally 12 μm-16 μm.

7. The secondary battery according to any one of claims 1 to 6, characterized in that: The particle size distribution of the graphite material (Dv90-Dv10) / Dv50 is 1.20-1.70, and can be optionally 1.35-1.

60.

8. The secondary battery according to any one of claims 1 to 7, characterized in that: The specific surface area of ​​the graphite material is 0.6 m 2 / g-1.5m 2 / g, optional 0.8m 2 / g-1.4m 2 / g.

9. The secondary battery according to any one of claims 1 to 8, characterized in that: The oil absorption value of the graphite material is less than or equal to 45 ml / 100 g, and can be optionally 25 ml / 100 g-44 ml / 100 g.

10. The secondary battery according to any one of claims 1 to 9, characterized in that: The graphitization degree of the graphite material is 88%-93%, and can be optionally 89%-92%.

11. The secondary battery according to any one of claims 1 to 10, characterized in that: The compaction density of the negative electrode film layer is 1.40 g / cm 3 -1.55g / cm 3 and / or, The surface density of the negative electrode film layer is 7 mg / cm 2 ~14mg / cm 2 .

12. The secondary battery according to any one of claims 1 to 11, characterized in that: The graphite material is artificial graphite.

13. The secondary battery according to any one of claims 1 to 12, characterized in that: The conductive agent includes at least one of conductive carbon black, acetylene black, graphite, graphene, carbon nanotubes, and carbon nanofibers.

14. An electrical device comprising the secondary battery according to any one of claims 1 to 13.

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